Pneumocystis jirovecii

Atypical yeast - human only host - opportunistic pathogen in immunocompromised - cannot be cultured in vitro

Taxonomy

• Previously thought to be a protozoan → assigned to fungal kingdom following phylogenetic studies of the rRNA
• Previously called Pneumocystis carinii → later found that Pneumocystis from humans is quite different to the species from other animals. The name Pneumocystis jirovecii was given to the species from humans; Pneumocystis carinii continues to be used for the species found in other animals
  • PCP - previously Pneumocystis Carinii Pneumonia. Now PneumoCystis Pneumonia
  • Consensus in 2001 to continue using PCP rather than PJP to describe clinical disease, but not everyone doing this: It is still PCP that can stand for Pneumocystis pneumonia: Appeal for generalized use of only one acronym | Medical Mycology | Oxford Academic
• Atypical fungus – has cholesterol instead of ergosterol in its cell membrane → not susceptible to many antifungals
• Unable to cultured in vitro
• Exists in 2 forms during its life-cycle:
  • Trophic form = metabolically active – found within lungs of infected host – lacks a cell wall
  • Cystic form = dormant, thick-walled structure – allows organism to survive outside host – has beta-D-glucan in the cell wall
• Both forms are present during its life-cycle – transmission primarily occurs through the cystic form

Epidemiology

• Ubiquitous among humans worldwide - acquisition is via airborne route
• Colonises human upper respiratory tract - it is likely that humans undergo frequent cycles of clearance and recolonisation
• Cluster outbreaks in hospitals have been reported suggesting some person-person transmission

Clinical

| Site | - Transiently colonises URT of adults and children

• Causes Pneumoc*ystis jirovecii* pneumonia (PCP) in patients with T cell deficiencies | | --- | --- | | Risk Factors | - HIV infection
• SOT
• Patients with cancer / haematological malignancy
• Congenital immunodeficiencies
• Prolonged corticosteroid use
• Use of immunosuppressive drugs (e.g. treatment of malignancy or prevention of transplant rejection) | | Clinical syndromes | Three clinical forms:
• Asymptomatic / pauci-symptomatic URT infection
• P. jirovecii pneumonia (PCP)
• Extra-pulmonary disease - very rare = mostly in advanced HIV

P. jirovecii pneumonia (PCP) is the most important clinical syndrome Presentation varies depending on underlying condition:

• HIV patients: insidious onset of fever, dyspnoea, non-productive cough and reduced exercise tolerance over days-weeks
• Non-HIV patients: acute onset fever, dyspnoea, non-productive cough and severe hypoxia
• Infants: cyanosis with respiratory distress
• CXR: diffuse bilateral interstitial ground-glass infiltrates - can be lobar/nodular
• CT: central ground-glass consolidation with peripheral sparing may be seen | | Pathogenic mechanisms | Pneumocystis jirovecii has a preference for infecting the lung in at-risk individuals. Microscopic examination reveals that Pneumocystis attaches to type I alveolar epithelium, which allows the fungus to transition from its small trophic form to the larger cystic form. Adherence of Pneumocystis to alveoli is not the singular cause of diffuse alveolar damage, but rather it is the host's inflammatory response that causes significant lung injury and impaired gas exchange, leading to hypoxia and possibly respiratory failure.  |

Extrapulmonary sites